Conventionally, the temperature of plasma has been thought to be roughly determined by the type of gas generating the plasma, the flow rate of gas, the quantity of energy applied, the method of generating the plasma, the atmosphere in a plasma generating chamber, and the like.
However, from the perspective of application to various fields, enabling the temperature of plasma to be controlled over a wider temperature range is being demanded. For example, in surface treatments using a conventional plasma apparatus, reaction speeds and treatment results are controlled through control of the temperature of the object to be treated (such as a substrate when treating a semiconductor). However, when methods in which the temperature of the object to be treated is controlled are used, a problem occurs in that the objects that can be treated and the like become limited.
In particular, there has recently been demand for lower plasma temperatures. Therefore, some attempts at lowering the temperature of plasma have been made by reducing energy supplied to plasma gas by increasing the flow rate of gas injected into the plasma in relation to energy supplied to the plasma generating chamber. Alternatively, the quantity of energy injected into the plasma is reduced. However, significant temperature reduction could not be achieved.
For example, reduction of the temperature of plasma has been attempted by using pulsed power supply and intermittently supplying the plasma with electric power, thereby reducing the total quantity of energy added to the plasma (to a very small quantity of 0.2 W to 3 W), when generating the plasma. In addition, an attempt has been made in which a discharge electrode is cooled. However, this attempt too aims to suppress “temperature rise” in the electrode and the plasma (refer to Non-patent Literature 1).
Furthermore, to lower the temperature of plasma, helium gas having high heat conductivity is used as plasma gas, heat generated in the plasma is released by being transmitted to the gas, electric power required for plasma generation is minimized, and power supply to the plasma is intermittently performed, thereby reducing the quantity of energy added to the plasma as a total (refer to pages 235, 236, and 245 of Non-patent Literature 2).
Moreover, attempts have been made to “not increase the plasma temperature at all” by pulse operation, power reduction, and increased flow rate of gas. However, these attempts all suppress temperature rise by “the temperature of the gas to be supplied”.    Non-patent Literature 1: The 35th IEEE International Conference on Plasma Science (ICOPS 2008) Oral Session 1E on Monday, June. 16, 09:30-12:00 Conference Abstracts, 2D4 TOXICITY OF NON-THERMAL PLASMA TREATMENT OF ENDOTHELIAL CELLS    Non-patent Literature 2: Micro-/Nano-Plasma Technology and industrial Applications, CMC Press, Dec. 27, 2006